1. Field of the Invention
The present invention relates to an exhaust purification device of an engine.
2. Description of the Related Art
Known in the art is an internal combustion engine having a first catalyst arranged in an exhaust manifold of the engine and having a second catalyst arranged in an engine exhaust passage downstream of the first catalyst (see Japanese Unexamined Patent Publication (Kokai) No. 6-93902). By arranging the first catalyst in the exhaust manifold in this way, the first catalyst is activated at a relatively early timing after engine start-up and therefore the exhaust gas can be purified from a relatively early timing after engine start-up. In this case, it is preferable to have the first catalyst activated at an even earlier timing so as to purify the exhaust gas at a further earlier timing after engine start-up. Therefore, in this internal combustion engine, for a while after engine start-up, the fuel injection timing is delayed and the ignition timing delayed so as to raise the exhaust gas temperature and thereby enable the first catalyst to be activated as early as possible.
Once the first catalyst is activated, however, it remains activated while the engine is operating. Therefore, the considerable amount of reducing components in exhaust gas such as unburned hydrocarbons or carbon monoxide is made to oxidize by the first catalyst at all times. In other words, the amount of reducing components in the exhaust gas flowing to the second catalyst does not become that large. This may appear to be preferable when seen from the viewpoint of purification of the exhaust gas. In actuality, however, the second catalyst sometimes requires a large amount of reducing components. In such a case, the problem arises that a sufficient amount of reducing components cannot be supplied to the second catalyst.
For example, since the second catalyst is arranged downstream of the first catalyst, it is lower in temperature than the first catalyst and therefore, unlike the first catalyst, tends to become deactivated in state even if once activated. Preventing the second catalyst from becoming deactivated in state requires that the second catalyst be heated. The most suitable method for heating the second catalyst is to have the reducing components of the exhaust gas such as unburned hydrocarbons or carbon monoxide be oxidized on the second catalyst and thereby cause the generation of heat of oxidation reaction in the second catalyst. Therefore, the exhaust gas flowing into the second catalyst must contain a large amount of reducing components.
Accordingly, if a considerable portion of the reducing components in exhaust gas is constantly being oxidized by the first catalyst and therefore the amount of the reducing components in the exhaust gas flowing into the second catalyst no longer becomes that large as in the case of the above internal combustion engine, the second catalyst easily becomes deactivated in state. If the second catalyst becomes deactivated in state, a considerable amount of reducing components such as unburned hydrocarbons or carbon monoxide is released into the atmosphere. That is, the problem arises that if the first catalyst is provided upstream of the second catalyst, the amount of unburned hydrocarbons and carbon monoxide exhausted into the atmosphere conversely ends up increasing. Therefore, when the second catalyst requires a large amount of reducing components, the amount of reducing components made to oxidize by the first catalyst has to be reduced.
An object of the present invention is to provide an exhaust purification device of an engine which is capable of effectively reducing an amount of unburned hydrocarbons and carbon monoxide discharged to the outside air.
According to the present invention, there is provided an exhaust purification device of an engine having an exhaust passage, comprising a first catalyst arranged in the exhaust passage; a second catalyst arranged in the exhaust passage downstream of the first catalyst; judging means for judging if the engine is in an operating state where reducing components in an exhaust gas should mainly be made to oxidize by the first catalyst or if the engine is in an operating state where reducing components in the exhaust gas should mainly be made to oxidize by the second catalyst; and oxidation ratio controlling means for making a ratio of oxidation of reducing components in the exhaust gas at the first catalyst increase when it is judged that the engine is in an operating state where reducing components in the exhaust gas should mainly be made to oxidize by the first catalyst and for making the ratio of oxidation of reducing components in the exhaust gas at the first catalyst decrease when it is judged that the engine is in an operating state where reducing components in the exhaust gas should mainly be made to oxidize by the second catalyst.